NEW YORK (AP) — This year has seen a record number of cases of a mysterious paralyzing illness in children, U.S. health officials said Monday.

It’s still not clear what’s causing the kids to lose the ability to move their face, neck, back, arms or legs. The symptoms tend to occur about a week after the children had a fever and respiratory illness.

No one has died from the rare disease this year, but it was blamed for one death last year and it may have caused others in the past.

What’s more, Centers for Disease Control and Prevention officials say many children have lasting paralysis. And close to half the kids diagnosed with it this year were admitted to hospital intensive care units and hooked up to machines to help them breathe.

The condition has been likened to polio, a dreaded paralyzing illness that once struck tens of thousands of U.S. children a year. Those outbreaks ended after a polio vaccine became available in the 1950s. Investigators of the current outbreak have ruled out polio, finding no evidence of that virus in recent cases.

The current mystery can be traced to 2012, when three cases of limb weakness were seen in California. The first real wave of confirmed illnesses was seen in 2014, when 120 were reported. Another, larger wave occurred in 2016, when there were 149 confirmed cases. So far this year, there have been 158 confirmed cases.

In 2015 and 2017, the counts were far lower, and it’s not clear why.

The condition is called acute flaccid myelitis, or AFM. Investigators have suspected it is caused by a virus called EV-D68. The 2014 wave coincided with a lot of EV-D68 infections and the virus “remains the leading hypothesis,” said Dr. Ruth Lynfield, a member of a 16-person AFM Task Force that the CDC established last month to offer advice to disease detectives.

But there is disagreement about how strong a suspect EV-D68 is. Waves of AFM and that virus haven’t coincided in other years, and testing is not finding the virus in every case. CDC officials have been increasingly cautious about saying the virus triggered the illnesses in this outbreak.

Indeed, EV-D68 infections are not new in kids, and many Americans carry antibodies against it.

Why would the virus suddenly be causing these paralyzing illnesses?

“This is a key question that has confounded us,” said the CDC’s Dr. Nancy Messonnier, who is overseeing the agency’s outbreak investigation.

Experts also said it’s not clear why cases are surging in two-year cycles.

Another mystery: More than 17 countries have reported scattered AFM cases, but none have seen cyclical surges like the U.S. has.

When there has been a wave in the U.S., cases spiked in September and tailed off significantly by November. Last week, CDC officials said the problem had peaked, but they warned that the number of cases would go up as investigators evaluated — and decided whether to count — illnesses that occurred earlier.

As of Monday, there were 311 illness reports still being evaluated.

This year’s confirmed cases are spread among 36 states. The states with the most are Texas, with 21, and Colorado, 15.

But it’s not clear if the state tallies truly represent where illnesses have been happening. For example, the numbers in Colorado may be high at least partly because it was in the scene of an attention-grabbing 2014 outbreak, and so doctors there may be doing a better job doing things that can lead to a diagnosis.

For an illness to be counted, the diagnosis must include an MRI scan that shows lesions in the part of the spinal cord that controls muscles.

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The Associated Press Health & Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.